The Tickford Racing driver timed his run to perfection in the happy hour at the end of the half-hour session, as everyone came to grips with the new track surface.
Randle had just put his Castrol Mustang on top as Macauley Jones clattered into the wall at Turn 8 and rolled to a stop at Turn 9.
That brought out the red flag with a minute left to run, race control opting to not restart the session.
That left Randle on top ahead of Brodie Kostecki and Cam Waters.
"The red flag definitely came out at a good time, straight after I did my lap," said Randle. "I don't know how many people did a lap.
"But it's credit to the whole team. The Castrol Racing Mustang was on song.
"A lot has changed. The layout hasn't changed, but the grip levels [have], and we started with a bit of oil on the track.
"The hardest thing for me was braking for the hairpin. You start braking on the old surface and half way through it changes to the new surface. That's somewhere I need to step up the marker.
"But it's a good start for tomorrow."
Shane van Gisbergen ended up fourth having spent two stints on top earlier in the session. He was among the drivers who didn't get a chance to improve due to the red flag.
Another was Scott Pye, who led the way with 10 minutes to go, but ended up back in sixth behind Andre Heimgartner.
Bryce Fullwood was seventh ahead of Mark Winterbottom, while Lee Holdsworth ended up ninth ahead of Will Brown despite missing most of the session due to an oil leak.
The Adelaide 500 continues with a second practice session at 1:15pm local time tomorrow.
